1. Field of the Invention
The present invention generally relates to a surface acoustic wave device used as a branching filter for, for example, cell phones and, in particular, to a surface acoustic wave demultiplexer having a first surface acoustic wave filter and a second surface acoustic wave filter which have different resonant frequencies and a trap including a one-port surface acoustic wave resonator connected to one of the surface acoustic wave filters.
2. Description of the Related Art
In recent years, sizes of telecommunications devices, such as cell phones, have been reduced. Various types of surface acoustic wave demultiplexers including a surface acoustic wave filter have been proposed since such surface acoustic wave demultiplexers are suitable for miniaturization in telecommunications devices.
For example, Japanese Patent No. 3224129 discloses a surface acoustic wave demultiplexer 101, whose plan view is shown in FIG. 8. The surface acoustic wave demultiplexer 101 includes first and second surface acoustic wave filter chips 103 and 104, respectively, having different resonant frequencies in a package 102. Herein, a center frequency of the surface acoustic wave filter chip 103 is lower than that of the surface acoustic wave filter chip 104. Each of the surface acoustic wave filter chips 103 and 104 includes a LiTaO3 substrate.
When the surface acoustic wave demultiplexer 101 is used in a cell phone, the first and second surface acoustic wave filter chips 103 and 104 are used as a transmission filter and a reception filter, respectively.
In the surface acoustic wave demultiplexer disclosed in Japanese Patent No. 3224129, the first and second surface acoustic wave filter chips 103 and 104 are composed of LiTaO3 substrates.
In general, in a surface acoustic wave demultiplexer, different characteristics are required for surface acoustic wave filters for transmission and reception. However, the surface acoustic wave filters with different characteristics affect each other greatly.
In particular, in telecommunications systems including a personal digital cellular (PDC) in which the pass bands for transmission and reception are significantly different, piezoelectric substrates composed of the same piezoelectric material sometimes do not provide sufficient performance.
If the first and second surface acoustic wave filters for reception and transmission are composed of piezoelectric substrates made from the same piezoelectric material, it is difficult to have superior characteristics for both surface acoustic wave filters.